Stellar dynamos with solar and antisolar differential rotations: Implications to magnetic cycles of slowly rotating stars

Karak, Bidya Binay; Tomar, Aparna; Vashishth, Vindya

doi:10.1093/mnras/stz3220

Cited by 20 publications

(21 citation statements)

References 79 publications

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“…The indication of tilt quenching as seen in Figure 4(a) gives an observation support of the following nonlinear quenching in the Babcock-Leighton α or in γ 0 routinely used to saturate the magnetic field growth in kinematic dynamo models (e.g., Choudhuri et al 1995;Dikpati & Charbonneau 1999;Chatterjee et al 2004;Karak et al 2019).…”

Section: Magnetic Quenching Of Tilt Anglesupporting

confidence: 58%

Magnetic field dependence of bipolar magnetic region tilts on the Sun: Indication of tilt quenching

Jha,

Karak,

Mandal

et al. 2019

Preprint

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The tilt of bipolar magnetic region (BMR) is crucial in the Babcock-Leighton process for the generation of the poloidal magnetic field in Sun. Based on the thin flux tube model of the BMR formation, the tilt is believed to be caused by the Coriolis force acting on the rising flux tube of the strong toroidal magnetic field from the base of the convection zone (BCZ). We analyze the magnetic field dependence of BMR tilts using the magnetograms of Michelson Doppler Imager (MDI) (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) and Helioseismic and Magnetic Imager (HMI) (2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). We observe that the distribution of the maximum magnetic field (B max ) of BMRs is bimodal. Its first peak at the low field corresponds to BMRs which do not have sunspots as counterparts in the white light images, whereas the second peak corresponds to sunspots as recorded in both type of images. We find that the slope of Joy's law (γ 0 ) initially increases slowly with the increase of B max . However, when B max 2 kG, γ 0 decreases. Scatter of BMR tilt around Joy's law systematically decreases with the increase of B max . The decrease of observed γ 0 with B max provides a hint to a nonlinear tilt quenching in the Babcock-Leighton process. We finally discuss how our results may be used to make a connection with the thin flux tube model.

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Section: Magnetic Quenching Of Tilt Anglesupporting

confidence: 58%

Magnetic field dependence of bipolar magnetic region tilts on the Sun: Indication of tilt quenching

Jha,

Karak,

Mandal

et al. 2019

Preprint

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“…In this work, we have offered some speculation on how stars like the Sun may have evolved from the times they were born to the time when they reached an age beyond that of the present Sun. We expect that the Sun is shortly before changing its rotation from solar-like to antisolar-like differential rotation where the equator rotates slower than the poles; see also Karak et al (2019) for recent mean-field modeling of such stars. Stars with antisolar differential rotation are also potential candidates for displaying superflares (Katsova et al 2018).…”

Section: Discussionmentioning

confidence: 99%

Magnetic field evolution in solar-type stars

Brandenburg

2019

Proc. IAU

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We discuss selected aspects regarding the magnetic field evolution of solar-type stars. Most of the stars with activity cycles are in the range where the normalized chromospheric Calcium emission increases linearly with the inverse Rossby number. For Rossby numbers below about a quarter of the solar value, the activity saturates and no cycles have been found. For Rossby numbers above the solar value, again no activity cycles have been found, but now the activity goes up again for a major fraction of the stars. Rapidly rotating stars show nonaxisymmetric large-scale magnetic fields, but there is disagreement between models and observations regarding the actual value of the Rossby number where this happens. We also discuss the prospects of detecting the sign of magnetic helicity using various linear polarization techniques both at the stellar surface using the parity-odd contribution to linear polarization and above the surface using Faraday rotation.

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“…We note that this anti-solar profile differs from the one used in Karak et al (2020). In this similar study using an α 2 Ω dynamo model, the value of the anti-solar coefficient Ω C /Ω Eq is kept at 0.93944 (as in the solar DR profile).…”

Section: Toroidal Magnetic Field Generationmentioning

confidence: 99%

“…Such models have been explored by Dubé & Charbonneau (2013) using mean-field coefficients extracted from 3D MHD prescriptions. Likewise, Karak et al (2020) have considered anti-solar models and found they could trigger magnetic cycles, but only with strong α-effect coupled with Ω-quenching. These authors obtained reversals in a α 2 Ω dynamo and argued that this could not be the case with classical kinematic αΩ models when considering a positive α-effect in the northern hemisphere.…”

Section: Introductionmentioning

confidence: 99%

Impact of anti-solar differential rotation in mean-field solar-type dynamos

Noraz

Brun

Strugarek

et al. 2022

A&A

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Context. Over the course of their lifetimes, the rotation of solar-type stars goes through different phases. Once they reach the zero-age main sequence, their global rotation rate decreases during the main sequence until at least the solar age, approximately following the empirical Skumanich’s law and enabling gyrochronology. Older solar-type stars might then reach a point of transition when they stop braking, according to recent results of asteroseismology. Additionally, recent 3D numerical simulations of solar-type stars show that different regimes of differential rotation can be characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby number (slow rotators). If this regime occurs during the main sequence and, in general, for slow rotators, we may consider how magnetic generation through the dynamo process might be impacted. In particular, we consider whether slowly rotating stars are indeed subject to magnetic cycles. Aims. We aim to understand the magnetic field generation of solar-type stars possessing an anti-solar differential rotation and we focus on the possible existence of magnetic cycles in such stars. Methods. We modeled mean-field kinematic dynamos in solar (fast equator, slow poles) and anti-solar (slow equator, fast poles) differential rotation, using the STELEM code. We consider two types of mean field dynamo mechanisms along with the Ω-effect: the standard α-effect distributed at various locations in the convective envelope and the Babcock-Leighton effect. Results. We find that kinematic αΩ dynamos allow for the presence of magnetic cycles and global polarity reversals for both rotation regimes, but only if the α-effect is saddled on the tachocline. If it is distributed in the convection zone, solar-type cases still possess a cycle and anti-solar cases do not. Conversely, we have not found any possibility for sustaining a magnetic cycle with the traditional Babcock-Leighton flux-transport dynamos in the anti-solar differential rotation regime due to flux addition. Graphic interpretations are proposed in order to illustrate these cases. However, we find that hybrid models containing both prescriptions can still sustain local polarity reversals at some latitudes. Conclusions. We conclude that stars in the anti-solar differential rotation regime can sustain magnetic cycles only for very specific dynamo processes. The detection of a magnetic cycle for such a star would therefore be a particularly interesting constraint in working to decipher what type of dynamo is actually at work in solar-type stars.

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Stellar dynamos with solar and antisolar differential rotations: Implications to magnetic cycles of slowly rotating stars

Cited by 20 publications

References 79 publications

Magnetic field dependence of bipolar magnetic region tilts on the Sun: Indication of tilt quenching

Magnetic field dependence of bipolar magnetic region tilts on the Sun: Indication of tilt quenching

Magnetic field evolution in solar-type stars

Impact of anti-solar differential rotation in mean-field solar-type dynamos

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